D1 and D2, two chloroplast proteins with apparent mol. wt of 32 000-34 000, play an important role in the photosynthetic reactions mediated by the membrane-bound protein complex of photosystem II (PSII). We have isolated and characterized an uniparental, non-photosynthetic mutant of Chlamydomonas reinhardtii and show that the mutation is in the chloroplast gene psbD, coding for D2. A 46 bp direct DNA duplication in the coding region of the mutant gene causes a frame-shift which results in a psbD transcript coding for 186 amino acid residues instead of the normal 352. The truncated D2 peptide is never seen, even after pulse-labeling, suggesting that the mutant protein is very unstable. In addition, little or no D1 protein is detected in this mutant although the gene and normal levels of mRNA for D1 are present in mutant cells. All other core PSII proteins are synthesized and inserted into the membrane fraction, but never accumulate. These results suggest that D2 contributes not only to the stabilization of the PSII complex in the membrane, but also may play a specific role in the regulation of the D1 protein, either at the translational or post-translational level.